Many marine animals use sound to sense and interact with their environment: for communication, navigation, finding prey and even for self defence. If the background noise levels are too high,masking or other potentially adverse effects on marine life will occur. Shipping is a major contributor to the background noise levels. The world's oceans are becoming increasingly noisy. The number of ships in the world's commercial fleet has roughly tripled between the 1948 and 2008 (see Hildebrand (1)), which equates to about 5 dB. On the other hand, anthropogenic noise in the oceans has been increasing by 3.3 dB per decade (Frisk (2)), or about 20 dB since the 1950s. In other words, ships are not only more numerous but individual vessels have become more noisy due to increased size, speed and delivered power. This leads to more cavitation, the main source of underwater radiated noise (URN). Therefore, shipping noise has received increasing regulatory attention. Class rules include noise limits for fishery and seismic research vessels because self noise can influence the operability of such vessels. Nowadays there also are URN-related class notations for other ship types, such as DNV-GLSilent-E, BV URN (NR 614) and RINA Dolphin, while ABS has also announced URN notations.